Field of the Invention
The invention relates to an integrated optical field width or beam spread transformer structured of several buffer layers on a substrate, the buffer layers being separated from each other by conductive layers, and an unclad rib wave guide of a constant rib width arranged on the upper buffer layer and having a tapered section at the interface between chip and fiber.
In integrated optics, cross-section changes occur when light guides are connected. The propagating light waves have to be subjected either to a widening or to a compacting change of field width in order to avoid abrupt transitions. In addition, the shape of the propagation images has to be matched when coupling photonic switching circuits to light waveguides. In integrated light waveguides these propagation images are of narrow lying elliptical shape and in fibers they are of rotationally symmetric shape. The elliptical shape cannot be avoided in unclad rib waveguides; but the propagating image in the waveguide may be enlarged by widening the field, so that losses from matching the fibre are low.
A number of publications has dealt with problems of optical waveguide couplings and with the coupling of waveguides to fibers. By way of example, German patent specifications DE 41 03 896 C1 and DE 41 42 850 A1 may be mentioned, see in particular FIGS. 3 through 5 and the associated description thereof. The state of the art upon which the invention is based, is known from "Proceedings 21st European Conference on Optical Communication ECOC '95, Brussels, Belgium, Paper TU P22, 1995, "Uncladded thickness tapers with InGaAsP/InP rib waveguides for efficient fibre-chip butt coupling", Morl et al. This beam spread transformer satisfies requirements as regards a low loss of 1 dB in mode matching as well as a cost-efficient fabrication. Because of the tapered section of the rib waveguide there occurs a shift of the propagation image in a vertical direction and a change in the beam spread in horizontal and vertical directions. This resulted in an optimization in respect of the TE polarization. The coupling loss between chip and fiber amounts to 2 dB as regards the TE component, but is higher by several dB for the TM component.